Who isn't afraid of being waken abruptly by a shaking building whose roof is caving in? Or a huge tsunami sweeping though your town? Earthquakes are terrifying. But what exactly are they? Why do earthquakes happen?

The Earth's surface isn't actually one continuous shell...

If you want to be picky about it, call it the lithosphere; it's made up a number of pieces called tectonic plates. They actually rest on the Earth's mantle: a deeper layer of rock which is neither solid nor liquid but somewhere in between. You can think of them like chunks of ice on an almost-frozen lake—they happily sit on the surface, but it's easy for them to move around.

...and it sometimes lets loose with sudden bursts of energy...

The problem is that, because the plates are free to move, they rub against each other. Because their edges aren't smooth, the plates can snag, and edges get stuck. That doesn't stop the rest of the plate moving, though, and as it does, the plate deforms slightly and stores up energy—like a spring stretching, except this spring weighs as much as a continent. Even the slightest deformation generates massive amounts of energy. When the relative movement becomes too much, and there's excess energy stored in the plate—BOOM—it's suddenly released: the "spring" snaps.

...that generate waves in the Earth's crust...

When all that energy is released, it takes the form of heat, rupturing, and seismic waves. The two big problems thrown up by 'quakes are ground rupture and vibration. Ground rupture is just what it sounds like: huge cracks, appearing seemingly out of nowhere, as the tectonic plates shift and wrench the upper surface of the Earth apart. More problematic, however, are the seismic waves that carry about 10 percent of the energy through the Earth's surface for miles—causing the ground to shake. Pictures fall off the wall, buildings collapse, and so on: Seismic waves can wreak havoc.

...which are measured using the Moment magnitude scale...

To describe how severe earthquakes are, scientists use the Moment magnitude scale—it replaced the Richter scale decades ago. It can be thought of as a measure of how much energy a quake releases. Magnitude is logarithmic, which means that at the same distance from the earthquake, the shaking will be 10 times as large during a magnitude 5 earthquake as during a magnitude 4 earthquake.

...have unpredictable effects...

But the numbers don't relate directly to the impact of the quakes—that's down to chance, as well as development and population density. To give some perspective, the Haiti earthquake of 2010—the third deadliest ever—was of magnitude 7.0, while 2011's Tōhoku 9.0 quake in Japan killed fewer but caused more expensive damage. A 7.1 magnitude in Fiji last year caused no fatalities.

...and happen extremely regularly...

It's estimated that 500,000 earthquakes occur every year—of which about 100,00 can be felt. The rest are too small, too deep, or to remote to register. But the big, nasty quakes occur far less often, and scientists can predict that roughly ten times as many earthquakes larger than magnitude 4 occur in a particular time period than earthquakes larger than magnitude 5. But that's just an approximation: Some years are worse than others.

... but are only occasionally catastrophic.

So, fortunately, we don't get bad earthquakes too often. The trouble is, when we're unlucky enough to have a serious earthquake hit a densely populated city, it's nearly impossible to avoid its destruction. All hell breaks lose. But early warning systems are improving—so with any luck, they're a natural phenomenon that will cause less destruction in the future.
Image: USACEpublicaffairs